1. Field of the Invention
The present invention concerns a power supply control device to which power from a power supply is provided externally and an information processing device using the power supply control device.
2. Discussion of Related Art
Recent years have witnessed the development of increasingly miniature and highly portable information devices other than desktop and laptop computers, such as mobile computers, electronic notebooks, digital cameras and storage devices and their interconnection now allows the exchange of information.
In conjunction therewith, there is also an growing dissemination of interface specifications allowing power to be supplied, as represented by the USB (Universal Serial Bus), IEEE1394.
In the past, peripheral device units have been kept small to avoid compromising portability and power has been supplied through an AC adapter or through a power supply terminal of a host personal computer interface connector.
However, the conventional method for supplying power has almost always been either an external supply using an AC adapter or the like, a battery or by way of an interface. That is because switching between two power supply inputs requires a switch for switching, or implementation of an “or” function using a diode, which have the following drawbacks, respectively.
Switching methods involving a switch have a cost disadvantage in that a switch must be installed and that power may be lost during switching. Further, if the mode of suppling power is interrupted, operation is not possible even if power can be otherwise supplied.
Methods using a diode are problematic in that, when the voltage on the interface side is high, switching does not actually occur even if an AC adapter is connected. In particular, IEEE1394 entails a relaxed specification of the power supply voltage. A broad specification with considerable latitude stipulating from 8V to 33V, or the lack thereof, as acceptable. This has required adoption of an AC adapter with a voltage higher than that maximum value and use of circuits with high withstand voltages, for example, and measures such as the provision of high-capacity capacitors.
FIG. 1 is a structural drawing of a conventional power supply control device in which 5V is supplied to an information processing unit. Power is supplied to this power supply control device from the power supply terminal of an interface connector and from an AC adapter by way of respective signal lines 1, 2. The power supply voltage supplied from the interface connector is set in the range of 8V-33V, according to the personal computer, or the like, connected thereafter. The power supply voltage supplied from the AC adapter is greater than 34V.
Backflow prevention diodes 8, 9 are provided in the signal lines 1, 2, preventing current flowing from one line from flowing back into the other line. Thus, the power supply voltage is supplied to a power supply circuit 4 from the line with the higher voltage.
The power supply circuit 4 comprises a field effect transistor 19, which performs switching, a flywheel diode 24, a coil 25, output voltage feedback resistors 13, 14, a decoupling capacitor 21, a smoothing capacitor 23 and a control circuit 26.
The output voltage from the power supply circuit 4 is divided by resistors 13, 14. Based on the result of a comparison to a predetermined reference voltage, the control circuit 26 issues instructions for switching to the field effect transistor 19 and standardizing feedback control is thereby carried out to provide a 5V output voltage.
In such a power supply control device, when the power is supplied from the interface signal line 1, and not from the adapter signal line 2, current flows from the interface signal line 1 into the decoupling capacitor 21 and is prevented from flowing into the adapter signal line by a backflow prevention diode 9.
When the power supply voltage from the interface signal line 1 is 33V, even though power is supplied from the adapter signal line 2, if a higher power supply voltage is not supplied, current flows from the interface signal line 1 to the decoupling capacitor 21.
Essentially, when 8V-33V is supplied from the interface, a voltage higher than the 33V, that is the maximum voltage supplied from the interface, is required to ensure that when an AC adapter is used, no power is received from the interface and the current received comes only from the AC adapter. Thus, accounting for error in precision, a voltage greater than 34V is required.
Because the relationship between the withstand voltage and the capacity of a capacitor is generally proportional, the smoothing capacitor within the AC adapter must also be increased. The slim likelihood of increasing the conversion efficiency of the power supply for conversion from 34V to 5V is also a problem.